1. Field of the Invention
The present invention relates to a PTC (Positive Temperature Coefficient) thermistor. More specifically, the present invention relates to a PTC thermistor having a thermistor body disposed between a pair of electrodes, whereas the thermistor body is constituted by a shaped article made of a thermoplastic resin, a low molecular weight organic compound, and an electrically conductive particle. The PTC thermistor of the present invention is favorably usable as a temperature sensor and an overcurrent protection device (e.g., an overcurrent protection device for a lithium ion battery).
2. Related Background Art
The PTC (Positive Temperature Coefficient) thermistor has a configuration comprising, at least, a pair of electrodes opposing each other, and a thermistor body disposed between the pair of electrodes. The thermistor body has a “positive resistance vs. temperature characteristic” in which its resistance value drastically increases as temperature rises.
By utilizing the above-mentioned characteristic, the PTC thermistor is used, for example, as a self-control type heat generator, a temperature sensor, a current limiting device, an overcurrent protection device, and the like for protecting circuits of electronic devices. From the viewpoint of the usage mentioned above and the like, the PTC thermistor is required to have a low resistance value at room temperature when not operating, a large change ratio between the resistance value at room temperature when not operating and that during operation, a small amount of change in resistance value when operated repeatedly (the difference between the resistance value at an initial stage of use and that after repeated operations), an excellent breaking characteristic, a low temperature generated by the device, and an ability to reduce its size, weight, and cost.
A common type of the PTC thermistor is equipped with a thermistor body made of a ceramics material. However, this type of PTC thermistor has been poor in the breaking characteristic, high in the temperature generated by the thermistor body, and hard to reduce its size, weight, and cost.
Therefore, in order to respond to the above-mentioned requirement for lowering the operation temperature and the like, a PTC thermistor of a type comprising a shaped article made of a thermoplastic region (polymer matrix) and an electrically conductive fine particle as a thermistor body (hereinafter referred to as “P-PTC thermistor” when necessary) has been under consideration.
Proposed as such a P-PTC thermistor is of a type in which a shaped article comprising electrically conductive fine particles dispersed into a crystalline polymer, which is a thermoplastic resin, is provided as a thermistor body (see, for example, the following patent literatures 1 and 2). The reason why the resistance value in such a P-PTC thermistor drastically increases at a predetermined temperature is presumed to be because the crystalline polymer constituting the thermistor body inflates as it melts, thereby cutting an electric conduction path constructed by the electrically conductive fine particles in the thermistor body.
Proposed as another example of P-PTC thermistor is of a type in which, for example, a shaped article obtained by mixing a crystalline polymer as a thermoplastic resin, a low molecular weight organic compound (having an average molecular weight of less than 2,000, for example), and electrically conductive fine particles (including carbon black as a main ingredient) is provided as a thermistor body (see, for example, patent literatures 3 to 13). This P-PTC thermistor seems to increase its resistance value when the low molecular weight organic compound melts.
Proposed as still another example of P-PTC thermistor is of a type in which a shaped article including an Ni metal powder having spiky protrusions as electrically conductive fine particles is provided as a thermistor body (see, for example, patent documents 14 and 15).
Patent Literature 1                U.S. Pat. No. 3,243,753        
Patent Literature 2                U.S. Pat. No. 3,351,882        
Patent Literature 3                Japanese Patent Publication No. SHO 62-16523        
Patent Literature 4                Japanese Patent Publication No. HEI 7-109786        
Patent Literature 5                Japanese Patent Publication No. HEI 7-48396        
Patent Literature 6                Japanese Patent Application Laid-Open No. SHO 62-51184        
Patent Literature 7                Japanese Patent Application Laid-Open No. SHO 62-51185        
Patent Literature 8                Japanese Patent Application Laid-Open No. SHO 62-51186        
Patent Literature 9                Japanese Patent Application Laid-Open No. SHO 62-51187        
Patent Literature 10                Japanese Patent Application Laid-Open No. HEI 1-231284        
Patent Literature 11                Japanese Patent Application Laid-Open No. HEI 3-132001        
Patent Literature 12                Japanese Patent Application Laid-Open No. HEI 9-27383        
Patent Literature 13                Japanese Patent Application Laid-Open No. HEI 9-69410        
Patent Literature 14                U.S. Pat. No. 5,378,407        
Patent Literature 15                Japanese Patent Application Laid-Open No. HEI 5-47503        